Commit 1362f4ea authored Feb 20, 2014 by Jan Kara

quota: Fix race between dqput() and dquot_scan_active()



Currently last dqput() can race with dquot_scan_active() causing it to
call callback for an already deactivated dquot. The race is as follows:

CPU1					CPU2
  dqput()
    spin_lock(&dq_list_lock);
    if (atomic_read(&dquot->dq_count) > 1) {
     - not taken
    if (test_bit(DQ_ACTIVE_B, &dquot->dq_flags)) {
      spin_unlock(&dq_list_lock);
      ->release_dquot(dquot);
        if (atomic_read(&dquot->dq_count) > 1)
         - not taken
					  dquot_scan_active()
					    spin_lock(&dq_list_lock);
					    if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags))
					     - not taken
					    atomic_inc(&dquot->dq_count);
					    spin_unlock(&dq_list_lock);
        - proceeds to release dquot
					    ret = fn(dquot, priv);
					     - called for inactive dquot

Fix the problem by making sure possible ->release_dquot() is finished by
the time we call the callback and new calls to it will notice reference
dquot_scan_active() has taken and bail out.

CC: stable@vger.kernel.org # >= 2.6.29
Signed-off-by: Jan Kara <jack@suse.cz>

parent 09ebb17a

fs/quota/dquot.c

+11 −3

Original line number	Diff line number	Diff line
		@@ -581,9 +581,17 @@ int dquot_scan_active(struct super_block *sb,
		dqstats_inc(DQST_LOOKUPS);
		dqput(old_dquot);
		old_dquot = dquot;
		/*
		* ->release_dquot() can be racing with us. Our reference
		* protects us from new calls to it so just wait for any
		* outstanding call and recheck the DQ_ACTIVE_B after that.
		*/
		wait_on_dquot(dquot);
		if (test_bit(DQ_ACTIVE_B, &dquot->dq_flags)) {
		ret = fn(dquot, priv);
		if (ret < 0)
		goto out;
		}
		spin_lock(&dq_list_lock);
		/* We are safe to continue now because our dquot could not
		* be moved out of the inuse list while we hold the reference */

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